1. Field of the Invention
The present invention relates to a printing system comprising a marking engine for applying a marking substance onto a sheet, and comprising a sheet conveying device, the sheet conveying device comprising a belt arranged to carry the sheets from an upstream end to a downstream end of a transport path, the belt having an array of perforations; an array of suction nozzles being arranged below the belt for sucking-in ambient air through the perforations of the belt; and at least one blow nozzle arranged at the downstream end of the transport path for blowing out a gas through the perforations of the belt.
2. Background of the Invention
Conveyor belts are frequently used for conveying sheets of paper, plastic film or the like through a sheet handling apparatus such as a printer or a copier. In many applications, e.g. an inkjet printer, it is important that the sheets are reliably held in a stable position on the belt so as to be conveyed through the apparatus with high accuracy, and/or the sheets are held flat on the belt, i.e. without forming any cockles. For that purpose, it is known to provide an array of suction nozzles below the belt, distributed along and across the transport path, so that a suction pressure is generated which draws the sheets against the belt and holds them safely in position.
When the sheets reach the downstream end of the transport path, they must reliably be separated from the belt so that they may either be discharged onto a tray or may be passed over to a subsequent conveying device, e.g. a set of conveyor rollers or another conveyor belt. Sometimes, the sheets tend to stick to the belt, because the sheet material is sticky or because a static charge is developed, which attracts the sheet against the belt. When the leading edge of the sheet is not separated quickly enough from the belt, a jam is likely to occur, or the sheet may be damaged.
Typically, the belt is deflected at a deflection roller at the downstream end of the transport path and the tendency of the sheet to stick to the belt may be reduced by reducing the radius of the deflection roller or by deflecting the belt at a relatively sharp knife-edge, so that the flexibility of the sheet is no longer sufficient for the sheet to follow the sharp turn of the belt. However, such a sharp turn may also cause strains in the belt itself, so that the lifetime of the belt is reduced. In order to mitigate this problem, it would be possible to increase the flexibility of the belt, which however, would make it even more difficult to assure the required transport accuracy and reliability.
It is generally known that the separation of the sheets from the belt may be assisted by blowing a stream of air against the leading edge of the sheet when it leaves the transport path at the downstream end.
US 2008/001347 A1 describes a sheet transport apparatus, in which a sheet is placed on an area of a transport belt and is held to the circulating transport belt by a negative pressure applied through the belt in a first suction unit. The transport belt is at least partially air permeable and, for example, has through openings in the shape of perforations. Electrical charges are applied to the transport belt in order to generate electrostatic holding forces between the transport belt and the sheet. In a printing module, only electrostatic forces hold the sheet, and a toner image is generated on the sheet. In a second suction unit, negative pressure is applied again, and a neutralizing unit neutralizes electrical charges on the transport belt. Downstream of the neutralizing unit, the sheet is lifted off the transport belt at a first blowing unit and is then further moved by the transport belt until it reaches an area of a guide roller. In this area, a second blowing unit is arranged within the hollow cylindrical body of the guide roller. Air is blown through openings in the hollow cylindrical body of the guide roller and through the transport belt.